It's because Riot doesn't seem to wish to support the map specifically, as they do most, if not all of the balancing based on SR. That's also why even though Twisted Treeline was revamped, it didn't really see much support afterwards, and that there's no Ranked Dominion queue yet.
The map design itself does lead to some champions being really overpowered and a pain to fight against, i.e. Kassadin, Kha'Zix, and Jayce.